![]() ![]() A secondary guidescope on top of the main imaging telescope requires extra counterweights. Placing another scope and its mounting hardware on top of the main imaging telescope adds extra weight to the payload in the worst place, making the mount work harder. Modern software can compensate for that to some extent, but in the end, you can’t cheat the physics. With the longer focal lengths of large telescopes, a small guidescope won’t have the resolving power to see the tiny movements that affect the main telescope and camera. Everything may feel solid to you, but it isn’t on a microscopic scale. With reflectors, the mirrors are not really fixed, and they tend to shift as the telescope tracks the sky. Connections between the guide and main scope can flex, as can (and likely will) either of the focusers of the scopes. The guidescope is controlling the entire system, but many of the other parts can move without the guider noticing. Some of the main limitations of using guidescopes are: While the guide camera thinks its tracking the sky correctly, there is no guarantee it’s seeing the exact same errors the main imaging camera and telescope are. It is possible to guide the mount with a guidescope mounted on top of the main imaging system, but this solution is not always accurate. ![]() The sharper image on the right is the result of perfect sky tracking. The image on the left was taken with poor tracking. We correct for these by “guiding” the mount. Since tolerances are in microns, deviations from perfect polar alignment, flexure of telescope optics and mechanics, and other factors all cause errors in the tracking rate. While this seems simple in theory, in practice, small discrepancies can spoil the perfect tracking we need. ![]()
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